The invention relates to a method of manufacturing ultrafine particles which are produced from a target by laser beam evaporation, and the use of the ultrafine particles thus manufactured.
Ultrafine particles are to be understood to mean herein, particles having diameters in the range from 1 to 100 nm; consequently, such particles are smaller than particles of conventional fine powders and larger than clusters of atoms (Chikara Hayashi, J. Vac. Sci. Technol. A5 (4), July/August 1987, pp. 1375-1384, and Physics Today, December 1987, pp. 44-51).
According to Hayashi in the above publication, ultrafine particles can be used:
in dyes, pigments, adhesives and catalysts, PA1 for ultrafine particles of Al.sub.2 O.sub.3, carburated tungsten, Si.sub.3 N.sub.4 and other heat and acid-resistant carbides and nitrides of active metals and rare earth metals in tools, ceramic and heat-resistant materials, PA1 for dispersion hardening, in which process ultrafine particles are dispersed in a host material and the mixture is sintered or exposed to the action of a catalyst, PA1 in aerosols for use in agriculture, forestry, military technologies and medicine, PA1 for ultrafine particles of magnetic alloys in magnetic recording materials, for example sound recording tapes and video tapes, and PA1 in microbiology. PA1 laser intensity I in the target surface PA1 absorptivity A(.lambda.) of the surface PA1 conductivity .kappa. of the material PA1 melting and boiling point of the material PA1 equilibrium partial pressures over the pure material as a function of the temperature. PA1 heated guide walls with an inflow of relatively cold gas between them PA1 heating the edge of a flow hose by exposure to laser radiation of the wavelength .lambda. in a suitable gas phase having a strong resonance absorption at .lambda. PA1 a cooled transport-gas nozzle and a laminar flow PA1 laser-gas cooling by stimulated emission from the excited state and a suitable wavelength at which only a a low degree of scattering by ultrafine particles takes place.
According to Hayashi, ultrafine particles are manufactured using induction-heated crucibles, in arc furnaces, Hayashi prefers the GEM method (Gas Evaporation Method), i.e. evaporation and condensation in a permanent gas.
Hayashi further describes the flotation and transport of ultrafine particles in a gas flow as well as a gas-coating method in which a high-speed gas flow entraining ultrafine particles impinges on a substrate at a low pressure of, for example, 1 hPa.
In U.S. application Ser. No. 4,619,691, a description is given of a method of manufacturing ultrafine particles by irradiating a surface of a material with a laser beam. When the radiation process is carried out in a properly selected gas atmosphere, for example, in oxygen, nitrogen, dichlorodifluoromethane, methane or propane, ultrafine particles having a desired composition are obtained, said composition being either the same as that of the irradiated material or different. The particle size distribution is adjusted through the pressure of the gas atmosphere, said pressure not exceeding 1000 hPa. When titanium is irradiated at a pressure of 1000 hPa, ultrafine particles having a diameter in the range from 5 to 65 nm are obtained, at a pressure of 130 hPa particles are obtained having a uniform diameter of 5 nm. The power density of the laser beam ranges between 10.sup.4 and 10.sup.7 W/cm.sup.2. Additional energy may be supplied to the irradiated material, for example, by means of an arc, a glow discharge or electron beams.
DE-A-3800680 describes a method of coating a substrate in a vacuum chamber, in which method a target is evaporated by means of a laser beam and the precipitate is used to coat the substrate, the laser beam being led into the vacuum chamber through an optical window, and the deposition of the precipitate on the window inside the vacuum chamber being precluded by means of a gas plume. The pressure in the vacuum chamber generally ranges between 10.sup.-2 and 10.sup.-6 hPa. An inert gas or, at least partly, a reactive gas which combines with the evaporated particles of the target to form a new chemical compound and, hence, becomes a constituent of the coating, can be selected as the gas for the gas plume. Materials having different melting and evaporation temperatures can evaporate side by side and be used as coating materials. The target is present either as a solid mixture or as a powder which is composed of a mixture of materials. A uniform wear and a regulation of the quantity evaporated per unit of time can be controlled by moving the target. By moving the target, always fresh materials are exposed to the laser beam and the burning of holes in the target is precluded. Ionization of the vapours to be used for coating and the accompanying inert gases is very advantageous for the transport of the evaporated particles from the target to the substrate. To enhance ionization, a low-pressure plasma can be formed at the location where evaporation of the target takes place and/or in the region of the substrate, said low-pressure plasma being maintained by, for example, a glow discharge. To further enhance the transport of evaporated particles on the substrate, the latter may be negatively charged.
The processing of laser material in a high vacuum as described in DE-A-3800680 results in particles of a very unspecific size, i.e. particles in the range above 1 .mu.m as well as molecules themselves. Further, a sufficiently high mass flow cannot be attained in said high vacuum.